1. Field of the Invention
The present invention relates to a system for diluting ultrapure chemicals which is intended to be used in the microelectronics industry. It relates more particularly to the dilution of ultrapure concentrated chemicals so as to obtain the desired purity at their point of use during various steps in the fabrication of semiconductors.
2. Description of Related Art
In integrated-circuit fabrication processes, contamination is in general a very important concern for those involved in developing a process for fabricating these semiconductors. In modern semiconductor fabrication processes, many steps consist of various kinds of cleaning steps. These cleaning steps may consist in removing organic contaminants, metallic contaminants, photoresists used during etching or inorganic residues from these products, circuit-etching residues, nascent oxides, for example of the SiO.sub.2 type, etc. At the present time, the cost of constructing and commissioning a semiconductor fabrication plant is typically of the order of one billion dollars and a large fraction of this cost relates to the various measures which are taken for monitoring particles, for cleaning and for monitoring contamination during the various semiconductor fabrication steps.
A major source of contamination consists of the impurities in chemical processes. Given that the cleaning operations are frequent and critical, contamination due to the chemistry of the cleaning must be scrupulously avoided.
Many different chemicals are used in the fabrication of semiconductors, such as aqueous ammonia, hydrochloric acid, hydrofluoric acid, silane, etc. For example, aqueous ammonia in the form of ammonium hydroxide is widely used in the various cleaning steps in the standard cleaning process called the "RCA" process. This process includes various steps and, in particular, first of all, solvent cleaning to remove most of the organic substances, using tetrachloroethylene or a solvent of this type, followed by base cleaning using a solution of hydrogen peroxide, ammonium hydroxide and ultrapure water typically in volume proportions respectively of about 1/1/5 or 1/2/7 and, finally, acid cleaning using a solution of hydrogen peroxide and hydrochloric acid in ultrapure water typically in respective volume proportions of 1/1/6 or 1/2/8.
For further details with regard to these various cleaning steps, reference should be made, for example, to the work by W. RUNYAN and K. BEAN, entitled "Semiconductors integrated circuit processing technology" (1990).
These various chemicals are often delivered to the factory gates in "bulk" form, the chemicals having a high concentration and an often-industrial purity, i.e. not suited to the purity necessary for the microelectronics industry, said products in concentrated or dilute form being purified so as to obtain ultrapure chemicals corresponding to the standards in the microelectronics industry.
An apparatus is known, from U.S. Pat. No. 5,552,660, for the blending and control of the concentration of a concentrated chemical in a diluent, which has the drawback in particular of using several measurements of the concentration of the diluted chemical, taking into account the lack of homogeneity of the solution delivered.